Semiconductor manufacturing, such as the fabrication of integrated circuits, typically entails the use of photolithography. A semiconductor substrate on which circuits are being formed, usually a silicon wafer, is coated with a material, such as a photoresist, that changes solubility when exposed to radiation. A lithography tool, such as a mask or reticle, positioned between the radiation source and the semiconductor substrate casts a shadow to control which areas of the substrate are exposed to the radiation. After the exposure, the photoresist is removed from either the exposed or the unexposed areas, leaving a patterned layer of photoresist on the wafer that protects parts of the wafer during a subsequent etching or diffusion process.
The photolithography process allows multiple integrated circuit devices or electromechanical devices, often referred to as “chips,” to be formed on each wafer. The wafer is then cut up into individual dies, each including a single integrated circuit device or electromechanical device. Ultimately, these dies are subjected to additional operations and packaged into individual integrated circuit chips or electromechanical devices.
During the manufacturing process, variations in exposure and focus require that the patterns developed by lithographic processes be continually monitored or measured to determine if the dimensions of the patterns are within acceptable ranges. The importance of such monitoring, often referred to as process control, increases considerably as pattern sizes become smaller, especially as minimum feature sizes approach the limits of resolution available by the lithographic process. In order to achieve ever-higher device density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features. Features on the wafer are three-dimensional structures and a complete characterization must describe not just a surface dimension, such as the top width of a line or trench, but a complete three-dimensional profile of the feature. Process engineers must be able to accurately measure the critical dimensions (CD) of such surface features to fine tune the fabrication process and assure a desired device geometry is obtained.
Typically, CD measurements are made using instruments such as a scanning electron microscope (SEM). In a scanning electron microscope (SEM), a primary electron beam is focused to a fine spot that scans the surface to be observed. Secondary electrons are emitted from the surface as it is impacted by the primary beam. The secondary electrons are detected, and an image is formed, with the brightness at each point of the image being determined by the number of secondary electrons detected when the beam impacts a corresponding spot on the surface. As features continue to get smaller and smaller, however, there comes a point where the features to be measured are too small for the resolution provided by an ordinary SEM.
Transmission electron microscopes (TEMs) allow observers to see extremely small features, on the order of nanometers. In contrast to SEMs, which only image the surface of a material, TEM also allows analysis of the internal structure of a sample. In a TEM, a broad beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site. Samples are typically less than 100 nm thick.
In a scanning transmission electron microscope (STEM), a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the substrate are collected by an electron detector on the far side of the sample, and the intensity of each point on the image corresponds to the number of electrons collected as the primary beam impacts a corresponding point on the surface.
Because a sample must be very thin for viewing with transmission electron microscopy (whether TEM or STEM), preparation of the sample can be delicate, time-consuming work. The term “TEM” as used herein refers to a TEM or an STEM and references to preparing a sample for a TEM are to be understood to also include preparing a sample for viewing on an STEM. The term “STEM” as used herein also refers to both TEM and STEM. One method of preparing a TEM sample is to cut the sample from a substrate using an ion beam. A probe is attached to the sample, either before or after the sample has been entirely freed. The probe can be attached, for example, by static electricity, FIB deposition, or an adhesive. The sample, attached to the probe, is moved away from the substrate from which it was extracted and typically attached to a TEM grid or sample holder using FIB deposition, static electricity, or an adhesive.
FIG. 1 shows a typical TEM sample holder 10, which comprises a partly circular 3 mm ring. In some applications, samples 20 can attached to one or more fingers 14 of the TEM sample holder by ion beam deposition or an adhesive. The sample extends from the finger 16 so that in a TEM an electron beam (shown in FIG. 13) will have a free path through the sample 20 to a detector under the sample when the plane of the TEM grid is perpendicular to the electron beam.
Several techniques are known for preparing TEM specimens. These techniques may involve cleaving, chemical polishing, mechanical polishing, or broad beam low energy ion milling, or combining one or more of the above. The disadvantage to these techniques is that they are not site-specific and often require that the starting material be sectioned into smaller and smaller pieces, thereby destroying much of the original sample.
Other techniques generally referred to as “lift-out” techniques use focused ion beams to cut the sample from a substrate or bulk sample without destroying or damaging surrounding parts of the substrate. Such techniques are useful in analyzing the results of processes used in the fabrication of integrated circuits, as well as materials general to the physical or biological sciences. These techniques can be used to analyze samples in any orientation (e.g., either in cross-section or in plan view). Some techniques extract a sample sufficiently thin for use directly in a TEM; other techniques extract a “chuck” or large sample that requires additional thinning before observation. In addition, these “lift-out” specimens may also be directly analyzed by other analytical tools, other than TEM. Techniques where the sample is extracted from the substrate outside the FIB system vacuum chamber (as when the entire wafer is transferred to another tool for sample removal) are commonly referred to as “ex-situ” techniques; techniques where the sample removal occurs inside the vacuum chamber are called “in-situ” techniques. In-situ lift out techniques are discussed in U.S. Provisional App. to Tanguay et al. for “S/TEM Sample and Method of Extracting S/TEM Sample” filed May 3, 2007 (which is hereby incorporated by reference but which is not admitted to be prior art by virtue of its inclusion in this Background Section)
In-situ lift out of a chunk-type sample is typically accomplished in the following successive steps, as shown in FIGS. 2-5. First, as shown in FIG. 2 to FIG. 5, the sample 20 is completely or partially separated from the substrate 21 by milling with a focused ion beam 22. This step is typically accomplished by using a dual beam FIB/SEM system such as the Expida™ 1255 DualBeam™ System, available from FEI Company of Hillsboro, Oreg., the assignee of the present invention. Next, as shown in FIG. 4, a microprobe tip 23 is attached to the sample by FIB-induced chemical vapor deposition. In the case of only partially separated samples, the sample is then completely freed by further FIB milling. This process typically results in a wedge-shaped sample 60, as shown in FIG. 6, which is approximately 10×5×5 μm in size. Top surface 62 is thus approximately 10 μm long×5 μm wide.
As shown in FIG. 5 and FIG. 7, the sample is then transported by the attached microprobe to a TEM sample holder 24. Transporting the sample typically does not change its orientation, so its top surface will still be oriented perpendicular to the plane of the TEM sample holder. The sample 20 is attached to the sample holder 24 (again with FIB-induced CVD) and then end of the sample where the microprobe 23 is attached is cut free. This sequence of steps is illustrated in FIG. 8 to FIG. 10.
At this point, the sample is thinned into an electron-transparent thin section, either in the same FIB system or after transfer to a second FIB system. Sample thinning is shown by FIGS. 11-13. The sample can then be imaged with an electron beam 25 in a TEM or STEM as shown in FIG. 14. A typical dual beam FIB/SEM has the SEM column oriented normal to the sample (at 90 degrees) and the FIB column at an angle of approximately 52 degrees. Because the FIB should be roughly perpendicular to the top surface of the sample during the thinning process (and thus parallel to the desired face of the thinned sample) and the SEM should be perpendicular to the sample face during STEM imaging, it is often necessary to change the sample orientation and reposition the sample between the thinning and imaging steps. As a result, on many prior art systems it is necessary to break vacuum in order to reposition the sample between the thinning and imaging steps.
Further, it is often desirable to image the sample during milling using the SEM. Imaging using SEM or STEM during sample thinning allows the sample thickness and location of the feature of interest within the sample to be monitored directly. STEM imaging can be used even when the sample surface is at an angle to the electron beam (as would be the case when the sample is oriented toward the ion beam during milling) by compensating for the angle mathematically. SEM imaging can also be used to monitor sample in the same fashion that cross-sections for sub-100 nm features are measured by a CD-SEM. The use of top-down SEM imaging to control FIB thinning is discussed in U.S. Provisional App. 60/853,183 by Blackwood et al. for “Method for S/TEM Sample Analysis” (which is hereby incorporated by reference but which is not admitted to be prior art by virtue of its inclusion in this Background Section).
Although typical tilting sample stages are used in many FIB/SEM systems, such stages typically have a maximum tilt of approximately 60 degrees. This is obviously not sufficient to allow the sample to be rotated so that the SEM is perpendicular to the sample face (at 90 degrees).
Pivoting sample stages with a rotational range of more than 90 degrees are known. One such “flipstage” arrangement is described in U.S. Pat. No. 6,963,068 to Asselbergs et al. for “Method for the manufacture and transmissive irradiation of a sample, and particle-optical system,” which is hereby incorporated by reference. This type of stage is commercially available as the Flipstage™ from FEI Company of Hillsboro, Oreg., the assignee of the present invention.
Although flipstage systems provide a number of advantages, such systems are expensive and not easily added to existing FIB/SEM systems. What is needed is an improved method for TEM sample preparation and analysis that allows the sample to be correctly repositioned for FIB milling, SEM/STEM imaging from above, and STEM imaging through the sample and can be used in a Dual Beam FIB-STEM system without a flip stage.